Amplifier



Feb. 6, 1945. D. E. MAXWELL 2,369,066

' AMPLIFIER Filed Dec. 29, 1945 ,5 mgl.

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7| 69 W Donald E.Maxwell,

RELM-lvs mpg/T slsNAl. -Db. H S ttorheythe amount of negative feedbackto Patented Feb. 6, 1945 AMPLIFIER Donald E. Maxwell, Scotia, N. Y.,assignor to General Electric Company, a, corporation of New YorkApplication December 29, 1943, Serial No. 516,063

10 Claims.

My invention relates to amplifiers and particularly to amplifiersprovided with negative feedback-circuits and controls for determiningprovide automatic level or gain control.

It is an object of my inventionto provide an amplifier having negativefeedback and an improved arrangement for controlling the amount offeedback to provide automatic gain control.

'I'he features of my invention which I believe to be novel are setIforth with particularity in the appended claims. My invention itself,however, both as to its organization and method of operation togetherwith further objects and advantages thereof may best be understood byreference to the following description taken in connection with theaccompanying drawing in which Fig. 1 is a diagrammatic representation ofone form of amplifier embodying my invention; Fig. 2 is a diagram of thebridge circuit in the feedback path orf the amplifier of Fig. 1; andFig. 3 is a graph representing certain operating characteristics of theamplifier of Fig. 1.

Referring now to Fig. 1, the amplifier comprises three resistance,coupled push-pull amplifier stages, I0, II, and I2, which may, forexample, be the speech input amplifier for a broadcasting stationtransmitting equipment. The signal to be amplified is supplied to aninput transformer I3 and is impressed on the first amplifier stage IIJand is then amplified in turn -by the stages II and I2 and supplied toan output transformer I4; the secondary of the transformer I4 may beconnected to supply a, transmitter or other utilization equipment in theusual manner. The device I5 includes a cathode I1, a gridor controlelectrode' I8 and an anode I9. The device I6 includes a cathode 20,control electrode 2I and an anode 22. The cathodes I'I and 20 areconnected by resistances 23, 24 and 25, the mid-portion of resistance 24being connected to ground through an adjustable contact. The outputs ofelectron discharge devices 21 and 28 of the push-pull stage I0 arecoupled to the vgrids I8 and 2I, respectively, through capacitors 35 and36 and resistors 29 and 30. Grid I8 is connected to cathode I1 throughresistors 29, 3|', and 23, while grid 2| is connected to cathode 20through resistors 30, 32, and 25. A capacitor 33 acts to maintain thejunction of resistors 29 and 3I at audio frequency ground potential,while a capacitor 34 acts likewise to maintain the junction of resistors30 and 32 at audio frequency ground potential. Resistors 23 and 25 actas biasing resistors for devices I5 and I6, respectively. A feedbackypath is provided between the output of the amplifier stage I2 and thecathode circuits of the devices I5 and I6 in order to provide negativefeedback and limit the gain of the amplifier.

- The stage I2 comprises electron discharge devices 31 and 38 havinganodes 39 and 40 connected to opposite ends of the primary 0ftransformer I4, the direct vcurrent anode voltage being supplied to acenter tap of the primary from a suitable source such as a battery 9.The feedback circuit comprises leads 4I and 42 connected respectively tothe anodes 39 and 40 and connected across one diagonal of a bridgecircuit 43, the other diagonal of which is connected across theresistance 24. Since the output of the stage I2 is coupled acrossresistors in the cathode leads of the stage II, audio frequency currentflowing through the feedback path produces a degenerative voltage in thecathode cirythe two halves of which are connectedin series throughresistors 45 and 46 to form two opposite arms of the bridge, the othertwo arms of which are formed by resistors 41 and 48. The arrangement ofthe bridge is more clearly indicated in Fig. 2 in which the two halvesor electron discharge devices of the tube 44 connected in series withthe resistances 45 and 46 are indicated as separate tubes 44a and 44h,respectively. The device 44a comprises a cathode 50, a grid or controlelectrode 5I and an anode 52 and the device 44h comprises a cathode 53,a grid or control electrode 54 and an anode 55. The grids 5I and 54 areconnected together and through resistances 56 and 58 and a parallelresistance network 59 to the direct current source 9. The resistance 58is adjustable with respect to the network 59 so that the bias on thegrids 5I and 54 may be varied. This bias is adjusted so that the `bridgeis balanced provided that the output signal does not exceed apredetermined maximum intensity; in other words, when the output signaldo'es not exceed a" certain predetermined intensity, no degenerativevoltage is nal intensity becomes increasingly negative. An amplifierincluding an electron discharge device 6I is connected across the outputof the transformer i4 and amplies the output signal and impresses itupon a rectifier including an electron discharge device or diode 62having cathodes 63 and anodes 64. The load circuit of the diode 62comprises a resistance 65, the network 59 from an adjustable contact 66tothe resistance 58, and the resistance 58 bypassed by a condenser 60.The diode 62 is biased by adjustment of the contact 66 so that the diodewill not pass current until a signal of at least a predeterminedintensity7 is supplied from the device 6|; however, as soon as thisvalue of signal is exceeded current passes through the device 62 and thebias on the grids and 54 is changed; this unbalances the bridge, thedegree orf unbalance of the bridge depending upon the intensity of thesignal impressed across the rectifier 62. Since the audio frequencycurrent flowing through the bridge produces a voltage in the resistor-24 in phase with the voltage on the grid of the respective one of thedevices I5 and I6, the amount of degeneration is proportional to thedegree of unbalance of the bridge which in turn is proportional to theintensity of the output signal. The operating characteristics of theamplifier are indicated in Fig. 3, the curve 69 of which shows relativeinput signal plotted against the output level in decibels. The zerovalue of relative input signal has been taken as the valueibelow whichthe bridge 43 is balanced. 'Ihe output signal thus varies directly inproportion to the input signal below this value, however, as soon as thecontrol circuit operates to unbalance the bridge, the output level isdecreased and is maintained substantially constant until it reaches avalue indicated at the bend on the curve 69, at this point the devies44a and 44h have reached cutoff bias and are effectively open circuited.After this point has been reached the amplifier acts as an ordinary idegenerative amplifier with a constant value of negative feedback equalto that at the point 10. Beyond Athe point 'l0 the output signal againvaries directly in proportion to the input signal as indicated in Fig. 3by the straight line parallel to that portion of the curve below thezero decibel relative input signal line. It will be noted that theportions of the curve indicated at a and lying between zero relativeinput signal and the point 'l0 provides substantially constant outputlevel, the total variation over the range of operation as indicated at bbeing less than three decibels. A curve 1| vin Fig. 3 indicates thepercentage distortion of the signal over the range of relative inputsignals. It will be noted from this curve that percentage distortionremains below one percent over the operating range a of the amplifier. Y

The resistors 45 and 46 employed in the opposite arms of the bridge 43in series with the two halves of the device 44 have been found to renderthe bridge very stable Iand minimize signal distortion. Furthermore, thefeedback loop includes only two resistance lcapacity coupled stages sothat the phase shift around the loop can be 180 only at zero frequencyand at infinite frequency and within the range of frequencies normally.encountered the phase shift is much less than 180. Consequently, the180 phase shift necessary for oscillation is not attained and parasiticoscillations are prevented;

the circuit is unconditionally stable for all degrecs off feedback.

This amplifier circuit has the advantage that ordinary triode tubes `maybe employed throughout and there is no need for especially matched tubesin the automatic gain control circuit. The

circuit is completely balanced and, therefore, Y

very high speed gain reduction is made possible without overshooting andinstability. Furthermore, this type of automatic gain control circuithas very low distortion because of the distortion reducing properties ofa negative feedback loop. The circuit can be operated at higher audiolevel than conventional automatic gain control circuits and ytherebyminimizes difculties arising`from noise, microphonics, and othercauses..

While I have ldescribed a particular embodiment of my invention, otherapplications will readily be apparent to those skilled in the art. I donot, therefore, desire my invention to be limited to the particularcircuit arrangement illustrated and described and I intend by theappended claims to cover all modifications that fall within the spiritand scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. An automatic gain control system including an electronic amplifierhaving a feedback path connected between the output and the inputthereof, a bridge circuit connected in said feedbackpath and havingelectron discharge devicesin two arms thereof, means for biasing saiddevices to balance said bridge circuitl when the output signal intensityof said amplifier is below a predetermined value to render said feedbackpath substantially balanced, and means dependent upon intensities of theoutput signal above said predetermined value and arranged to produce aunidirectional potential to vary the bias on said devices and unbalancesaid bridge for controlling the amount of feedback in accordance withsignal intensity.

2.` An automatic gain control system including an electronic amplifierhaving a feedback circuit connected between the output and inputthereof, said feedback circuit being arranged t0 supply energy from saidoutput circuit to said input circuit in degenerative phase, a bridgecircuit connected in said feedback circuit and having electron dischargedevices in two arms thereof, means for biasing said devices to balancesaid bridge when the output signal intensity of said amplifier is belowa predetermined value to render said feedback circuit substantiallybalanced whereby the intensity of the output signal increases linearlywith respect to that of the input signal up to said predetermined value,and means dependent upon intensities of the output signal above saidpredetermined value and arranged to produce a unidirectional potentialto vary the bias on said devices and unbalance said bridge forcontrolling the amount of feedback in accordance with signal intensityto maintain the output signal intensity substantially constant abovebias on said devices and unbalance said bridge.

fo;` controlling the amount of feedback in accordance with signalintensity.

4. An automatic gain control system including f an electronic amplifiercomprising at least one pair of electron discharge devices connected inpush-pull relation, each of said devices having an anode and a cathodeand a control electrode,

.an output circuit connected between the anodes of a pair of saiddevices, an input circuit connected between the control electrodes of apair of said devices, a feedback circuit including a bridge connectedbetween said output circuit and said input circuit, resistance elementscomprising two arms of said bridge and electron discharge devicescomprising the other two arms of said bridge, one diagonal of saidbridge being connected between said anodes in said output circuit andthe other diagonal of said bridge being connected across the inputcircuit of said amplifier, means for biasing said devices in said bridgeto balance said bridge when the output signal intensity of saidamplifier is below a predetermined value to render said feedback circuitinoperative, and means dependent upon intensities of the output signalabove said predetermined value for changing the bias of said devices ofsaid bridge to unbalance said bridge and vary the feedback in accordancewith output signal intensity.

5. An automatic gain control system including an electronic amplifiercomprising at least one pair of electron discharge devices connected inpush-pull relation, each of said devices having an anode and a cathodeand a control electrode, an output circuit connected between the anodesof a pair lof said devices, an input circuit connected between thecontrol electrodes of a pair of said devices, a resistance connectedbetween the cathodes of said last mentioned pair of devices, a feedbackcircuit comprising a bridge having one diagonal connected across said.output circuit and its other diagonal connected across a portion ofsaid resistance, and means dependent upon the intensity of the signal atsaid output for varying the balance of said bridge to control the amountof feedback through said feedback circuit.

6. An automatic gain control system including of the signal at theoutput of said system above a predetermined value for varying the ybiason said devices in said bridge -to unbalance said bridge and render saidfeedback circuit operative and for controlling the amount of feedback inaccordance with the signal intensity.

7. An automatic gain control system including an electronic amplifiercomprising at least one pair vof electron discharge vdevices connectedin push-pull relation, each. of saiddevices having an anode and acathode anda control electrode, an output circuit connected ,between theanodes of a pair of said devices, an input circuit connected between thecontrol `electrodes of a pair of said devices, a resistance connectedbetween-the cathodes of ysaid last mentioned pair of devices, adegenerative feedback circuit comprising a bridge having one diagonalconnected across said output circuit and its other diagonal connectedacrossa portion of said resistance, said bridge including' two armshaving electron discharge devices therein, means for biasing saiddevices in said bridge to balance said bridge and render said feedbackinoperative whereby the output signals of said amplifier vary linearlywith respect to the input signals up to a predetermined` value of outputsignal intensity, and means 4including a rectifier associated with theoutput circuit and dependent upon intensity of the output signal abovesaid predetermined value for varying the bias on said devicesin saidbridge to unbalance'said bridge and render said feedback circuitoperative to maintain the intensity of the signals at said outputsubstantially constant.

8. An automatic gain control system including an electronic amplifierhaving a feedback circuit connected between the output and the inputthereof, said circuit including a bridge connected therein andcomprising resistance elements forming two arms thereof and electrondischarge devices forming the other two arms thereof, said bridge beingconnected in said feedback circuit so that one diagonal of the bridge isconnected across the output of said amplifier and the other diagonalthereof is connected across the input.

means comprising a rectifier associated with the an electronic amplifiercomprising at least one pair of electron discharge devices connected inpush-pull relation, each of said devices having an anode and a cathodeand a control electrode, an output circuit connected between the anodesof a pair of said devices, an input circuit connected between thecontrol electrodes of a pair of said devices, a resistance connectedbetween the cathodes of said last mentioned pair of devices, a feedbackcircuit comprising a bridge having one diagonal connected across saidoutput circuit and its other diagonal connected across a portion of saidresistance, said bridge including two arms having electron dischargedevices therein, means for biasing said devices in said bridge tobalance said bridge and render'said feedback circuit inoperative, andmeans dependent upon intensities ance elements forming two arms thereofand l output of said amplifier for producing a unidirectional potentialproportional tothe intensity of the output signal and means forrbiasingsaid rectifier to prevent operation thereof when said output signal isbelow said predetermined value.

9. An automatic gain control system including an lelectronic amplifierhaving a feedback circuit connected between the output and the inputthereof, said feedback circuit including a bridge connected therein andcomprising resistelectron discharge devices forming the other two armsthereof, said bridge being connected in said feedback circuit so thatone diagonal of the and a control electrode,l means for biasing thecontrol electrodes of said devices to balance said bridge when theintens-ity of the signal at the output of said amplier is below apredetermined value to render said feedback circuit substantiallyinoperative, and means dependent upon ance element and an electrondischarge device in series therewith, said bridge being connected insaid feedback circuit so that one diagonal of the bridge is connectedacross the output of intensities of the output signal above saidpredetermined value and arranged to produce a unidirectional potentialto vary theV bias simultaneously on the control eiectrodesof both saidde- 'vices and unbalance said bridge for lcontrolling the amount offeedback in accordance with signalintensity.

10. An automatic gain control system including an electronicr amplifierhaving a feedback circuit connected between the output and the inputthereof, said Vcircuit including a bridge having two arms eachcomprising a, resistance fv element and two arms each comprising aresist-

